I've considered doing partigyle brews myself but haven't yet so I can't speak with any authority. However, I can relay what I've read regarding this brewing tecnique:

Your primary beer should consist of first runnings only. This will result in a smaller batch of beer than you typically make. The small beer is then made from the sparge sometimes adding an additional specialty grain into the concoction.

However, since we are homebrewers, I don't see any reason you can't experiment a bit. Maybe instead of running off the 6.5 gallons you typically do, run off just 5 and shoot for about a 4 gallon batch. Continue your sparge and collect your small beer until the gravity of the runnings gets too low. You should be able to make some sort of a dry stout with this, however it is unlikely it will be as good as a specifically formulated dry stout.

It will depend a lot on your crush and the dead space in your tun, but on my system I'd get around 60% efficiency from 6.5 gallons of no-sparge first runnings from 22# of grain, giving me 1.072 for an approximately 1.086 beer.

I could do a 4.5 gallon partigyle to get around 1.034 wort for 3.75 gallons of 1.040 Dry Stout. It would probably finish pretty dry, too, so I might cap with a little Crystal Malt, like Bobcat suggests.

I used Kai Troester's Batch Sparge Calculator to get these numbers. It models my system very well, but probably wouldn't apply well to fly sparging half of the first 6.5 gallons, if you were considering that.

Yes you can.. We recently did a 10 gal batch of california common using 25lbs of grain and after reaching our boil amount we continued the fly sparge and got an extra 4 gal of wort that achieved 1.040 gravity which we used to make a light beer.. pay attention to the gravity of the running testing frequently to achieve the gravity you want... if the gravity starts to get too low, say 1.020 of the runnings stop there and boil up your second batch... we do this all the time and usually get an extra 3 gal batch or so, especially from our barley wines... It's generally called a small beer...